1 Linux Serial Console
2 3To use a serial port as console you need to compile the support into your
4kernel - by default it is not compiled in. For PC style serial ports
5it's the config option next to "Standard/generic (dumb) serial support".
6You must compile serial support into the kernel and not as a module.
7 8It is possible to specify multiple devices for console output. You can
9define a new kernel command line option to select which device(s) to
10use for console output.
11 12The format of this option is:
13 14 console=device,options
15 16 device: tty0 for the foreground virtual console
17 ttyX for any other virtual console
18 ttySx for a serial port
19 lp0 for the first parallel port
20 ttyUSB0 for the first USB serial device
21 22 options: depend on the driver. For the serial port this
23 defines the baudrate/parity/bits/flow control of
24 the port, in the format BBBBPNF, where BBBB is the
25 speed, P is parity (n/o/e), N is number of bits,
26 and F is flow control ('r' for RTS). Default is
27 9600n8. The maximum baudrate is 115200.
28 29You can specify multiple console= options on the kernel command line.
30Output will appear on all of them. The last device will be used when
31you open /dev/console. So, for example:
32 33 console=ttyS1,9600 console=tty0
34 35defines that opening /dev/console will get you the current foreground
36virtual console, and kernel messages will appear on both the VGA
37console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.
38 39Note that you can only define one console per device type (serial, video).
40 41If no console device is specified, the first device found capable of
42acting as a system console will be used. At this time, the system
43first looks for a VGA card and then for a serial port. So if you don't
44have a VGA card in your system the first serial port will automatically
45become the console.
46 47You will need to create a new device to use /dev/console. The official
48/dev/console is now character device 5,1.
49 50(You can also use a network device as a console. See
51Documentation/networking/netconsole.txt for information on that.)
52 53Here's an example that will use /dev/ttyS1 (COM2) as the console.
54Replace the sample values as needed.
55 561. Create /dev/console (real console) and /dev/tty0 (master virtual
57 console):
58 59 cd /dev
60 rm -f console tty0
61 mknod -m 622 console c 5 1
62 mknod -m 622 tty0 c 4 0
63 642. LILO can also take input from a serial device. This is a very
65 useful option. To tell LILO to use the serial port:
66 In lilo.conf (global section):
67 68 serial = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
69 703. Adjust to kernel flags for the new kernel,
71 again in lilo.conf (kernel section)
72 73 append = "console=ttyS1,9600"
74 754. Make sure a getty runs on the serial port so that you can login to
76 it once the system is done booting. This is done by adding a line
77 like this to /etc/inittab (exact syntax depends on your getty):
78 79 S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
80 815. Init and /etc/ioctl.save
82 83 Sysvinit remembers its stty settings in a file in /etc, called
84 `/etc/ioctl.save'. REMOVE THIS FILE before using the serial
85 console for the first time, because otherwise init will probably
86 set the baudrate to 38400 (baudrate of the virtual console).
87 886. /dev/console and X
89 Programs that want to do something with the virtual console usually
90 open /dev/console. If you have created the new /dev/console device,
91 and your console is NOT the virtual console some programs will fail.
92 Those are programs that want to access the VT interface, and use
93 /dev/console instead of /dev/tty0. Some of those programs are:
94 95 Xfree86, svgalib, gpm, SVGATextMode
96 97 It should be fixed in modern versions of these programs though.
98 99 Note that if you boot without a console= option (or with
100 console=/dev/tty0), /dev/console is the same as /dev/tty0. In that
101 case everything will still work.
102 1037. Thanks
104 105 Thanks to Geert Uytterhoeven <geert@linux-m68k.org>
106 for porting the patches from 2.1.4x to 2.1.6x for taking care of
107 the integration of these patches into m68k, ppc and alpha.
108 109Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000
110